Interpretive Summary: Cultured fish suffer diseases caused by waterborne parasites. One way to combat these parasites is by addition of chemicals to the culture water to kill the parasites. Chemicals capable of killing parasites may also be toxic to the fish that are in the water. To assess the toxicity of one chemical, potassium permanganate (which has been used many years for this purpose), fish were exposed to three concentrations of the chemical and then measurements were made of physiological changes known to be associated with stress. Mortality was scored; manganese content of liver and gill tissue was measured; concentrations of cortisol, glucose and chloride in blood plasma were measured; red blood cell counts were performed and whole blood packed cell volumes were measured. Mortality due to exposure to potassium permanganate appears to be due to loss of salts from the blood. Manganese is not absorbed by the fish yet large changes occur in the composition of the blood leading to salt loss and cardiac failure. At concentrations of the chemical that would normally be used for disease control, minimal signs of stress were observed suggesting the chemical is quite safe to use for disease treatment.

Technical Abstract:
Juvenile channel catfish (Ictalurus punctatus) were exposed to low (0.438 mglL), intermediate (1.315 mg/L), or high (2.19 mg/L) concentrations of waterborne KMnO4 for 36 h to determine the toxicity of the chemical. After exposure the fish were observed during a 14 d recovery period. Gill, liver, and blood samples were collected before exposure, at 12, 24, and 36 h during exposure, and at 48 h intervals for 14 d thereafter. Analysis of whole homogenized gill tissue showed a transient increase in manganese content that quickly disappeared once exposure was discontinued; the increase is thought to have been due to surface adsorption and not to internalization of manganese. Fish exposed to the intermediate and high concentrations of KMnO4 experienced 9 and 50.6% mortality, respectively. Plasma cortisol was elevated over ten-fold in the high exposure group and a statistically significant, though lower, increase in plasma cortisol occurred in the intermediate exposure group. Both plasma chloride and osmolality were significantly reduced in the intermediate and high exposure groups but were unchanged in the low exposure group. Packed cell volumes (PCV) of whole blood rose significantly in response to intermediate and high concentrations of KMnO4. Mortality may have been the result of blood electrolyte depletion as indicated by increased PCV's and loss of chloride and osmolality. All parameters measured, except PCV in the high exposure group) were indistinguishable from unexposed controls within 48 h after exposure was discontinued. At the exposure concentration most like that employed in a disease treatment (the low exposure group) no changes were observed in any parameter measured suggesting that potassium permanga- nate may be safely used as a disease therapeutant for channel catfish.